In cases where the attenuating function, polarizing function, scattering function, blocking function, and the like of irradiated light including laser beams and natural light are required, devices operating based on different principles have been conventionally allocated for each of these functions. As a result, products corresponding to these functions have been manufactured by different manufacturing processes for each of these functions.
For example, a linear polarizer, circular polarization plate, or the like is employed in a liquid-crystal element (LCD) to control optical rotation and birefringence in the display. A circular polarization plate is employed in an organic electroluminescent element (OLED) to prevent reflection of external light. Conventionally, iodine is widely employed in these polarizing plates (polarizing elements) as a dichroic substance. However, since iodine sublimates readily, it affords inadequate heat resistance and light resistance when employed in a polarizing element. The quenching color becomes deep blue, rendering the element less than ideal as a colorless polarizing element over the entire region of the visible spectrum.
Polarizing elements in which dichroic substances are employed as organic dyes have been investigated. However, these organic dyes present problems in that they yield only polarizing elements with a dichroic property that is considerably inferior to that of iodine. In particular, in LCDs in which the rotational or birefringent property of light is utilized as a display principle, the polarizing element is an important constituent component. In recent years, the development of new polarizing elements has progressed with the goal of enhancing display performance and the like.
One method of achieving this is, for example, in the same manner as in a polarizing element containing iodine, to dissolve or adsorb a dichroic organic dye (dichroic dye) in a polymer material such as a polyvinyl alcohol and stretch the film in one direction to orient the dichroic dye. However, this method presents problems in that processes such as stretching entail time and effort.
Accordingly, other methods have recently garnered attention. As such a method, in Nonpatent Reference 1, the intermolecular interaction of organic dye molecules on a substrate such as glass or a transparent film is utilized to orient the dichroic dye and form an anisotropic dye film such as a polarizing film. However, the method described in this reference is known to present a problem in terms of heat resistance.
Further, a wet film-forming method is employed to achieve the above orientation of the dichroic dye through the intermolecular interaction of organic dye molecules on a substrate such as glass or a transparent film. When manufacturing an anisotropic dye film by such a wet film-forming method, it is necessary to employ a dye that is suited to the process of wet film formation as well as to the high dichroism of the dye molecules as the dye employed in the dye film. Examples of wet film formation methods are methods of depositing and orienting the dye on a substrate and methods of controlling the orientation of the dye. Accordingly, many of the dyes that can be employed in polarizing elements obtained by the above conventional stretching process are unsuited to wet film forming methods. Patent References 1 to 3 propose materials that are suited to the process set forth above. However, even though suited to this process, these materials present a problem in that they are incapable of exhibiting a high degree of dichroism.
a) The fact that the angle formed between the moment of inertia and the transition moment is small, b) the fact that they have a high molecular aspect ratio, and the like are known means of obtaining highly dichroic dyes (Nonpatent References 2 and 3). Azo dyes, anthraquinone dyes, and the like are known to have dye skeletons that meet such characteristics. In particular, polyazo dyes having multiple azo groups in the long axis direction of the molecule are employed as skeletons manifesting desired and high light absorption coefficients. Symmetrical polyazo dyes with a single skeleton bonded through a single bond or linking group are an example of a means of conveniently synthesizing a polyazo dye with a high aspect ratio. A number of such dyes are described in Nonpatent Reference 4.
However, symmetrical polyazo dyes normally exhibit problems in the form of high crystallinity and poor solubility.
Additionally, Nonpatent Reference 5 describes a method of forming an anisotropic film having a high degree of order by coating an oligophenylene-vinylene compound of specific structure on an alignment film as a method of obtaining an anisotropic film using molecules other than dyes. However, when employed as a dye, this compound has a short wavelength and tends to fade when irradiated with light.